Process of preparing photoconductive layers for electrophotography



United tes Patent Q 3 128 204 PRGCESS or PnErAiuNh; rrrorocoNnUcTrvE LAYERS FOR ELECTROPHUTOGRAPHY Gustav Schaurn and Hildegard Haydn, Leverirusen, Germany, assignors to Agfa Airtiengeselischaft, Leverhusen, Germany, a corporation of Germany No Drawing. Filed Nov. 13, 1957, Ser. No. 696,024 Claims priority, appiication Germany Nov. 14, 1956 5 Claims. Ci. 117-421) The present invention relates to photoconductive layers for electrophotography.

For the production of photoconductive layers for electrophotography it is known to use certain inorganic or organic photoconductive compounds. Examples of such compounds are sulfur and selenium; oxides, sulfides and selenides of zinc, cadmium, mercury, antimony, bismuth, and lead, and also anthracene and anthraquinone. These substances are applied to an electrophotographic plate, especially a plate consisting of metal or another material, for example paper, or to a sheet or foil produced from a film-forming plastic. If necessary, the photoconductive substance can be held dispersed in an elec trically isolating layer-forming binder. Such layers are produced by means of coating solutions in which the binders are either dissolved or dispersed and in which the electroconductive substances are dispersed. Such photoconductive layers are disclosed, for instance, in United States Patents Nos. 2,297,691; 2,357,809; 2,727,807; 2,727,808; 2,735,784; and 2,735,785, and in French Patents Nos. 1,113,933; 1,122,275; 1,125,235; and 1,136,146.

The photoconductive layers which are produced from aqueous coating solutions in which the binders are dispersed suffer from the disadvantage that they readily take up moisture from the air owing to the presence of wetting agents and salts, so that the conductivity of the layer in the dark is often raised to such a degree that its image reproduction characteristics are impaired.

It has now been found that the dark conductivity of the photoconductive layers can be reduced substantially if waterproofing agents are added to the dispersions. These waterproofing agents can be of various types.

Suitable waterproofing agents are the following:

(a) paraffin Wax or similar high-molecular hydrocarbons which are preferably applied in the form of their aqueous emulsions; such emulsions are, for instance, disclosed in British Patents Nos. 477,830; 492,742; 469,642, and 627,356.

([2) Complex compounds of the Werner type in which a trivalent nuclear chromium atom is coordinated with an acyclic canboxylic acid (alkylcarboxy) group having at least ten carbon atoms, as they are disclosed in United States Patent No. 2,273,040. A suitable compound is for instance chromium stearoyl chloride.

(0) Fatty acids salts of polyvalent metals, more especially, fatty acids salts of aluminum, titanium, zirconium, and chromium.

(d) Metal compounds which are capable of hardening gelatin or other proteins, such as zirconium oxychloride, phosphotungstic acid, chrome alum, chromium acetate, chromic acid, alum, and cerous nitrate.

The water-repellent or waterproofing agents are preferably applied in the following quantities:

The parafiin emulsions when used as emulsions in 3,128,204 Patented Apr. 7, 1964 amounts of 20120%, the compounds quoted under ((7) and (c) in amounts of 15-30%, and the compounds disclosed under (d) in amounts of 0.014% as calculated on the binding agent.

As film-forming agents for the photoconductive layers in which the photoconductive substances are dispersed there may be used natural or synthetic resins or resinoids preferably such as are soluble in organic solvents, these film-forming agents being used in form of aqueous dispersions as disclosed, for instance, in French Patent No. 1,136,146. After coating, the layers produced from said dispersions may be subjected to a final condensation or final polymerization by heat treatment at temperatures of about -150" C. for about 2 to 30 minutes after being dried. Aqueous dispersions of polymeric substances which are soluble in organic solvents, such as melamineformaldehyde or urea-formaldehyde resins, xylene-formaldehyde resins, polymers based on vinyl chloride, vinyl idene chloride, vinyl ethers, acrylic esters, methacrylic esters, acrylic amides, such, for example, as methacrylic amide, aromatic vinyl compounds, such, for example, as styrene, isoolefins, such, for example, as isobutylene, copolymers based on the aforementioned compounds, such, for example, as copolymers of vinyl chloride and butyl acrylate, vinylidene chloride and butyl acrylate, synthetic elastomers, such, for example, as copolymers of butadiene and styrene, butadiene and acrylonitrile, and also copolymers of dienes with a preponderant proportion of styrene and/or acrylonitrile or other vinyl compounds, as well as polyamides, polyurethanes, polyesters of polycarboxylic acids and polyhydric alcohols, polycarbonates, cellulose esters, rubber and the like, are, for example, employed.

For the production of the photoconductive layers there may be used photoconductive inorganic or organic substances, for example sulfur and selenium; oxides, sulfides, selenides and iodides of cadmium, mercury, antimony, bismuth, thallium, moylbdenun, aluminum, lead or zinc; arsenic trisulphide; cadmium arsenide; lead chlorate; anthracene; benzidine; anthraquinone; acenaphthene; fluoranthene; naphthalene; chrysene; quinone; gallic acid; and microcrystalline paraffin wax. Terphenyl or p-diphenyl benzene, benzanthrone, 1,5-dicyanonaphthalene, 1,4-dicyanonaphthalene, aminophthalonitrile and nitrophthalonitrile are also particularly suitable.

The coating solution which is used for the production of the photoconductive layers comprises at least one of the aforementioned substances and a film-forming binding agent, the electric specific resistance of which must be higher than that of the photoconductive substance and preferably also higher than that of the layer support. The best results are obtained with binding agents the electric specific resistance of which amounts to at least 10 ohms per centimeter.

The quantitative ratios between the photoconductive substances and the binding agents may vary within wide limits. It is preferred to apply the photoconductive substance in amounts of 1 part per 0.3-2 parts by weight of binding agent, and in amounts of 5-60 grams per square meter of photoconductive layer.

As a support for the photoconductive layer there may be used paper or metal plates, such as zinc, aluminum, or brass plates. Furthermore, thin foils of cellulose hydrate, cellulose esters or of polyamide come into question.

.7 a The processing of the disclosed materials is carried out in the usual manner. The materials are first made sensitive to light by giving them an electrostatic charge on the coating side in the dark, for instance, by means of a corona discharge. The material is then exposed by any of the conventional photographic procedures. The latent image obtained is developed by applying a pigmented resin powder carrying an electrostatic charge which is opposite to that of the photoconductive layer.

Example 1 120 cc. of a 30% aqueous dispersion of a heat-hardenable melamine-formaldehyde resin,

110 cc. of water, and

40 g. of zinc oxide are thoroughly mixed in a ball mill and 1 cc. of a 3% zirconium oxychloride solution is added. The dispersion is cast on a paper or metal support and dried. This dispersion is not stable for very long, because condensation proceeds very rapidly. The dispersion must therefore rapidly be cast after adding the zirconium oxychloride.

Example 2 120 cc. of a 30% aqueous dispersion of a melamineformaldehyde resin as described in Example 1,

110 cc. of water 40 g. of zinc oxide, and

30 g. of a 10% aqueous parafin wax emulsion are thoroughly mixed in a ball mill, cast onto a support in the usual manner and dried. The paraffin wax emulsion can also be added just before casting.

Example 3 100 cc. of a 30% aqueous dispersion of a melamineformaldehyde resin as descibed in Example 1,

90 cc. of water,

60 g. of zinc oxide,

20 cc. of a 10% paraffin wax emulsion, and

7 cc. of 30% formalin are treated as above, it being advisable to add the formalin just prior to casting. Instead of formalin, a corresponding quantity of zirconium oxychloride or phosphotungstic acid may be added to the mixture.

In the foregoing examples there may be used instead of the melamine-formaldehyde resin a urea-formaldehyde resin which is heat hardenable.

Example 4 40 g. of zinc oxide are added to 50 cc. of a 35% aqueous emulsion of a copolymer of butadlene-acrylonitrile and styrene (40:40:20) after addition of 80 cc. of water. To this dispersion there are added 1.5 g. of chromium stearoyl chloride. After treating the dispersion for 2 hours in a ball mill, it is coated on a paper support and dried at 70 C.

Example 5 40 g. of zinc oxide are added to 100 cc. of a 35% aqueous dispersion of a copolymer of vinyl chloride, and acrylic acid and ethyl acrylate. To this dispersion there are added 25 cc. of an aqueous paraflin wax emul sion. After treating in a ball mill for 2 hours, 100 cc. of water and 10 cc. of ammonia are added. Thereafter the dispersion is cast onto a paper support and dried at 80 C.

What we claim is:

1. A process of producing an electrophotographic plate which comprises mixing together in a ball mill zinc oxide, an aqueous dispersion of a heat-hardenable melamineformaldehyde film-forming resinous binding agent in an amount between 0.3 and 2 parts by weight of the resin per part by weight of the zinc oxide, and an aqueous emulsion of paraffin wax in an amount between 2 and 12 percent by weight of the resinous binding agent based on sented by the following quantities:

(i) 40 grams of zinc oxide,

(ii) 120 cubic centimeters of an aqueous dispersion of a heat-hardenable melamine-formaldehyde resin containing 30 percent by weight of resin,

(iii) cubic centimeters of water, and

(iv) 30 grams of an aqueous emulsion of parafiin wax containing 10 percent by weight of paraffin wax.

3. A process of producing an electrophotographic plate which comprises mixing together in a ball mill zinc oxide, an aqueous dispersion of a heat-hardenable melaminedormaildehyde film-forming resinous binding agent in an amount between 0.3 and 2 parts of the resin per part of the zinc oxide, together with a waterproofing substance of the group consisting of (i) an aqueous emulsion of paraffin wax in an amount between 2 and 12 percent by weight of the resinous binding agent based on the paraffin wax content of the emulsion, and

(.ii) zirconium oxychlo-ride in an amount vbetween 0.1

and 1 percent by weight of the resinous binding agent, and subsequent-1y applying the said aqueous dispersion to a conductive support in such amount that the photoconduoting layer deposited therefrom contains between 5 and 60 grams of zinc oxide per square meter, and subsequently drying the said coated conductive support.

4. A process as defined in claim 3 in which the aqueous dispersion with which the conductive support is coated contains the following ingredients in proportions represented by the following quantities:

(i) 40 grams of zinc oxide,

(ii) cubic centimeters of an aqueous dispersion of a heat-hardenable melamine-formaldehyde resin containing 30 percent by weight of resin,

(iii) 110 cubic centimeters of Water, and

(iv) 1 cubic centimeter of an aqueous solution of zirconium :oxychloride containing 3 percent by weight of zirconium oxychloride.

5. A process as defined in claim 3 in which the aqueous dispersion with which the conductive suppont is coated contains the following ingredients in proportions represented by the following quantities:

(i) 60 grams of zinc oxide,

(ii) 100 cubic centimeters of an aqueous dispersion of la heat-hardenable melamine-formaldehyde resin containing 30 percent by weight of resin,

(iii) 90 cubic centimeters of water,

(iv) 20 cubic centimeters of an vaqueous emulsion of paraffin wax containing 10 percent by weight of paraffin wax, and

(v) 7 cubic centimeters of a 30 percent aqueous solution of fopmaldehyde.

References Cited in the file of this patent UNITED STATES PATENTS 2,273,040 Iler Feb. 17, 1942 2,287,348 Hayden June 23, 1942 2,391,621 Powell et a1. Dec. 25, 1945 2,394,009 Pollard Feb. 5, 1946 2,399,489 Landcs Apr. 30, 1946 2,663,636 Middleton Dec. 27, 1953 2,748,095 Kling May 29, 1956 2,875,054 Gr-iggs et a1 Feb. 24, 1959 (Either references on following page) 5 6 UNITED STATES PATENTS OTHER REFERENCES 2901348 Dessauer at 1959 Wainer: Photographic Engineering, vol. 3, N0. 1, pp. 2,959, 81. Kaucera NOV. 8, 1960 12 22 1952 Jun: 1961 Young et a l; R.C.A. Review, v01. XV., No. 4, 2,993,787 ugarmim u y 9 5 9 4 Decmber 2997387 Tamnbaum 1961 Mees: The Theory of the Phutographic Process, Mac- FOREIGN PATENTS mil lan (1954), pp. 73-84. 4 Belgium Dec. 30, 1955 Websters New ln temaltional Dictionary, 2nd Ed. 201,301 Australia May 19, 1956 10 (1954), p. 1771, aparafiin.

1,136,146 France May 9, 1957 

3. A PROCESS OF PRODUCING AN ELECTROPHOTOGRAPHIC PLATE WHICH COMPRISES MIXING TOGETHER IN A BALL MILL ZINC OXIDE, AN AQUEOUS DISPERSION OF A HEAT-HARDENABLE MELAMINE-FORMALDEHYDE FILM-FORMING RESINOUS BINDING AGENT IN AN AMOUNT BETWEEN 0.3 AND 2 PARTS OF THE RESIN PER PART OF THE ZINC OXIDE, TOGETHER WITH A WATERPROOFING SUBSTANCE OF THE GROUP CONSISTING OF (I) AN AQUEOUS EMULSION OF PARAFFIN WAX IN AN AMOUNT BETWEEN 2 AND 12 PERCENT BY WEIGHT OF THE RESINOUS BINDING AGENT BASED ON THE PARAFFIN WAX CONTENT OF THE EMULSION, AND (II) ZIRCONIUM OXYCHLORIDE IN AN AMOUNT BETWEEN 0.1 AND 1 PERCENT BY WEIGHT OF THE RESINOUS BINDING AGENT, AND SUBSEQUENTLY APPLYING THE SAID AQUEOUS DISPERSION TO A CONDUCTIVE SUPPORT IN SUCH AMOUNT THAT THE PHOTOCONDUCTING LAYER DEPOSITED THEREFROM CONTAINS BETWEEN 5 AND 60 GRAMS OF ZINC OXIDE PER SQUARE METER, AND SUBSEQUENTLY DRYING THE SAID COATED CONDUCTIVE SUPPORT. 